You can see the archive.php's title php code on twenty fifteen theme here:
<?php the_archive_title( '<h1 class="page-title">', '</h1>' ); the_archive_description( '<div class="taxonomy-description">', '</div>' ); ?>
I would like to add some custom text after category/tag title. Can I do that with archive.php? How to do that? In twenty fifteen theme both category and tag pages use archive.php.